Fasciculations caused by ALS are due to dying muscle tissue (actually necrotic). So, fasciculations due to ALS are always accompanied by muscle weakness and EMG changes.
In ALS they can begin in any muscle but usually it is the leg, but it can be the bulbar muscles, arms, etc. This will vary in a particular patient. In the early part of ALS they tend to be in specific muscles, more distinct, because the muscle is dying.
Fasciculations can be seen in a number of diseases, such as nemaline rod myopathy. So, if there is muscle weakness with EMG changes, then fasciculations are due to a specific disease.
Benign fasciculations are not due to dying muscle tissue, but irritated muscle membranes. They are not accompanied by muscle weakness, nor are there EMG changes.
When they occur in isolation, they are usually of the benign variety. Any and every neurologist will tell you this. Whether they occur in specific areas of the muscle is variable.
Benign fasciculations are usually more spread between muscle groups and can worsen with anxiety and fatigue. They can reoccur in the same muscle groups and also be variable. Since muscle is not dying in benign fasciculations, they tend to be spread out and in a variety of muscle groups.
Hope this helps and clarifies your doubts about the difference between "ALS fasciculations" and the "Benign" ones.